Ukraine crisis: Does Russia have a case?

Russia says it is acting in Ukraine to protect the human rights of its citizens. But what justification does it have for sending troops into Crimea?

What is Russia's claim to Crimea?
Its historical links with the peninsula go back to Catherine the Great in the 18th Century, when Russia conquered southern Ukraine and Crimea, taking them from the Ottoman Empire. In 1954, Crimea was handed to Ukraine as a gift by Soviet leader Nikita Khrushchev, who was himself half-Ukrainian. Only 10 years earlier, Joseph Stalin had deported Crimea's entire Tatar population, some 300,000 people, allegedly for co-operating with Hitler's Germany.

When Ukraine became independent in 1991, Russian President Boris Yeltsin agreed that Crimea could remain in Ukraine, with Russia's Black Sea fleet remaining at Sevastopol under lease. That lease was in recent years extended to 2042.

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Is there a legal basis for Russia's actions?
Under the 1994 Budapest Memorandum, the US, Russia, Ukraine and the UK agreed not to threaten or use force against the territorial integrity or political independence of Ukraine. They also pledged never to use economic coercion to subordinate Ukraine to their own interest.

Russia says its decision to send troops into Ukraine is necessary to protect Russian citizens.